1. Field of the Invention
This invention relates generally to air bearings and particularly to a self-aligning air bearing for rotationally supporting an extended rotatable disk drive servowriting apparatus.
2. Description of the Related Art
Success in the disk drive industry requires meeting market demand for increased density at reduced cost. Profitability requires rapid time-to-market. The hard disk drive industry is widely known for its short product life cycles, rapid improvement and innovation, and fierce price competition in established products. These factors require storage capacity improvements that can be quickly carried out in many disk drives during manufacture without slowing production.
Considerable recent research and development effort has been directed toward producing disk drives capable of storing ever more data. At the approach of the popular personal computer trend, in the early 1980's, disk drive data storage capacities of 10 to 20 Megabytes were considered adequate. Currently, disk drive capacities of 840 Megabytes are used in notebook computers, while disk drive capacities of 1.6 Gigabytes and more are seen in desktop microcomputers. The demand for higher capacity disk drives shows no signs of abating and ever newer storage-hungry operating systems and applications appear likely to continue to fuel demand.
A trivial way to increase disk storage capacity is merely to increase the size and/or number of disk surfaces available for storing data. However this runs contrary to consumer demand for smaller disk drive volume, which necessarily limits the size and number of available storage disks. In particular, there is a continuing demand for ever smaller disk drives arising from the consumers' appetite for ever smaller notebook and laptop computers with ever larger data storage capacity.
One recognized way to satisfy both the demand for smaller size and the demand for larger storage capacity is to increase the areal recording density of the disk surfaces used to store data. The areal recording density of a disk surface is equal to the product of track density, expressed in tracks per millimeter (tpmm) or tracks per inch (tpi), and the linear bit density for each track, expressed in bits per millimeter (bpmm) or bits per inch (bpi). The track spacing decreases as track density increases so that available track density is usually limited by the available accuracy and precision of the radial read/write head positioning mechanism. As track density increases, more accurate positioning of the head over the centerline of the track is necessary to avoid inadvertently reading or writing in the wrong track. So, overall, high disk drive storage capacities require closely-spaced narrow tracks having high linear bit densities and the capability to position the read/write heads with great precision.
Data storage density is practically limited by the performance capacity of the servo-track writer (STW) apparatus employed to record head-positioning servo-patterns on the data storage surfaces of rotary disks during disk drive manufacture. The servo-patterns contain information that specifies physical data-track spacing and sector timing. Such information is precisely placed on the data storage surfaces during manufacture so that a read/write head can be accurately positioned to read and write data during disk drive operation. As disk drive storage capacities increase, head-positioning error tolerances decrease, requiring corresponding reductions in servo-pattern errors.
To appreciate the necessary head positioning accuracy, considering a numerical example is useful. For a disk surface with a track density of about 80 tpmm (2000 tpi), the space allocable to each track (track pitch) is about 12.5 microns (500 microinches). A typical design goal in such an application is to provide a total operational head positioning accuracy of 10% (.+-.5%) of the track pitch, which limits positioning errors from all causes to less than about 1250 nanometers (50 microinches) in this example.
Available disk drive track density, which has historically increased about 60% every year, is now approaching 250 tpmm (6000 tpi). The necessary head positioning accuracy for 250 tpmm is about 4 microns (16 microinches). Practitioners long ago introduced closed-loop servo-positioning schemes that use head-positioning information written directly on the disk surfaces by a STW system during manufacture. During drive operation, a precision servosystem uses the recorded servo-positioning information to generate a feedback signal useful for properly placing the read/write head on the disk surface. Such servo-positioning information is called in the art a "servopattern." A feedback signal (servo data) generated in response to the servopattern read from the disk surface is used to drive an actuator motor that causes rotation about a pivot axis of an actuator arm to which read/write heads are attached, thereby positioning the heads with respect to the disk surface.
The recorded servopattern may, for instance, include a pattern of two analog burst signals, A and B, written between and spatially overlapping each data track pair. These servo signals are read by the positioned head and compared by the drive control circuitry to assess whether the head is centered on a data track (precisely between the two A and B servo burst signals). An error signal representing the difference in amplitude between the two servo burst signals is generated and used to drive the actuator in a direction that reduces the head position error. The servopattern may be either "embedded" on each disk surface or "dedicated" to one disk surface for use in head positioning for all other disk surfaces in the disk drive. Whether the servopattern is in embedded or dedicated, it must be precisely and accurately written by a STW system during manufacture before the disk drive can be used to store data.
The STW art can be appreciated with reference to, for instance, "Servo Writers . . . the Pros and Cons of Different Approaches" by Richard Freedland et at. (Proceedings of the Hewlett Packard Data Storage Symposium, 1991, Paper #7, pp. 7.1-7.19) and "Servowriters: A Critical Tool in Hard Disk Manufacturing" by Clayton Lee (Solid State Technology, May 1991, pp. 207-211). STW positioning accuracy is critical to hard disk manufacturing yields because even small servopattern position errors impair head positioning in modern high-density disk drives. Some portion of the operational head-positioning error is introduced as servopattern error by the STW itself because of alignment errors and imperfections in the recording system. These error contributions have until now been minor and practitioners in the art have merely tolerated the underlying STW imperfections. This is no longer acceptable with recent increases in track and bit densities.
Tolerable STW error contributions are typically limited to 10% of the entire operational tracking error budget for the disk drive. Recalling that total tracking error budget is typically 10% of track pitch, the STW system error contribution is then limited to 1% of the track pitch. In the above example, a disk drive wid a track density of about 80 tpmm (2000 tpi) has a track pitch of about 12.5 micrometers (500 microinches) and an operational head-positioning error budget of about 1250 nanometers (50 microinches). Accordingly, the STW system error contribution is limited to 125 nanometers (5.0 microinches). Similarly, STW system head-positioning error contributions are limited to 40 nanometers (1.6 microinches) in a disk drive having a track density of about 250 tpmm These STW positioning error limit calculations can be inverted to compute the maximum track density possible for a specified STW head-positioning error limit. For example, an STW system that contributes up to 125 nanometers (5.0 microinches) of head-positioning error cannot be used in the manufacture of disk drives having track densities over about 80 tpmm (2000 tpi), a value too low to satisfy existing market demands for high data storage densities.
Efforts to meet demands for greater storage density should not compromise existing capability to rapidly manufacture numerous disk drives. To avoid unacceptable losses in manufacturing volume, improvements in STW accuracy must be provided without reducing available rates of manufacturing throughput. Efforts of the industry to meet such needs have led to a well-known horizontally-oriented STW system capable of simultaneously writing servopatterns to more than one disk drive head-actuator assembly (HDA).
Reference is made to the above-cited Freedland and Lee references for a description of an existing horizontally-oriented STW system for multiple HDAs, which is now briefly described. Each of several HDAs is disposed within a separate sector of a flat table in the STW system Each HDA includes a read/write head fixed to the end of an actuator arm, which is disposed to pivot so that the read/write head follows an arcuate path about the actuator arm pivot axis. The actuator arm pivots responsive to an actuator motor during normal disk drive operation but is instead moved during STW operation by an engaging pin fixed to a rotary structure driven by a motor in the STW system A back bias current is applied to the actuator motor in each HDA to hold the actuator arm in place against the engaging pin. The rotary STW structure is supported by a base air bearing that defines a STW bearing axis about which the rotary STW structure rams. A retroreflector displacement sensor is fixed to the rotary STW structure for monitoring displacement during STW operation.
Rotation of the rotary STW structure moves each of several engaging pins in an arcuate path about the STW bearing axis. Engaging pin movement accordingly moves the corresponding HDA actuator arm about its pivot axis, which accordingly moves the attached read/write head in an arcuate path on a corresponding disk surface. Each engaging pin moves its read/write head by a distance that is precisely but indirectly measured by a laser transducing system, which includes a laser head, an interferometer, an optical receiver, and supporting electronics and also the retroreflector displacement sensor fixed to the rotary STW structure. The laser head sends a light beam through an interferometer where it is split into two beam, one beam to the retroreflector and back to the receiver and the other beam directly to the receiver. As the retroreflector moves, it changes the path length from laser head to receiver. In a well-known fashion, the receiver filters the arriving light through a polarizer and compares the phase difference between the direct beam and the reflected beam to create an electrical signal that includes retroreflector displacement information. If retroreflector displacement is mathematically related to read/write head displacement on a disk surface in a HDA, the electrical signal is then useful for precisely controlling the writing of a servopattern on the HDA disk surface.
Disadvantageously, in the horizontally-oriented STW system for multiple HDAs known in the art, each STW engaging pin and its corresponding HDA actuator arm rotate on different axes, producing unwanted angular displacement during STW operation that creates a "transmission error" in the STW head-positioning control system. Some head-positioning error is also contributed by the noise from frictional drag between pin and arm as the engaging pin pushes against its HDA actuator arm.
To solve some of these problems, the inventor created the vertically-extended rotary STW apparatus for multiple HDA servo-track writing to permit engaging pin and actuator arm axis alignment in the manner described in detail in the above-cited Szeremeta application and briefly hereinbelow with reference to FIG. 2. Szeremeta's vertically-extended rotary STW apparatus has a large height compared with its width, and therefore, unlike existing STW systems in the art, needs precisely-aligned rotational support at both the top and bottom ends to ensure rotary stability. As is known in the STW system art, frictional bearing noise contributes to head-positioning error and must be minimized by using a frictionless air bearing. Use of a single base air bearing is known in existing STW systems. However, any such frictionless bearing support added to the distal end of a vertically-extended rotary structure must also be precisely aligned with the STW bearing axis defined by the base bearing to minimize head-positioning error arising from bearing runout and to avoid component damage.
The problem of providing frictionless support for a rotary vertically-extended apparatus is a general problem with rotary machinery that is particularly critical in the STW art because of the sensitive head-positioning requirement. A mechanical component tolerance buildup in any system is known to skew rotary elements from a design axis. This is especially true in the STW art because clearances between rotational elements and stationary support elements, such as sleeves, are usually very small, particularly in frictionless bearings. A self-aligning bearing eliminates tolerance buildup problems but no self-alining bearing is known in the art that does not introduce significant frictional noise.
For example, U.S. Pat. No. 3,875,589 discloses a frictionless air bearing used to support a spindle that rotates a disk pack including a plurality of disks but the bearing is not self-alining. Also, U.S. Pat. No. 4,531,569 discloses a frictionless non-self-aligning air bearing combined with a rotary shaft for turning a recording disk pack. U.S. Pat. No. 5,193,084 discloses a frictionless axial air bearing used to support a recording disk turntable rotationally but the axial air bearing is combined with a non-frictionless radial bearing that alone is self-aligning.
There is accordingly no known source of a frictionless self-alining bearing, particularly a bearing offering both low friction and high alignment precision. Without such a self-aligning frictionless bearing, practitioners are obliged to accept either larger head-positioning errors from misaligned engaging pin axes or lower servopattern manufacturing rates. These unresolved deficiencies are clearly felt in the art and are solved by this invention in the manner described below.